Thick film conductor case compositions for LTCC tape

ABSTRACT

A thick film composition consisting essentially of:
         a) electrically conductive powder;   b) an inorganic binder wherein the inorganic binder is selected from TiO 2  and any compounds that can generate TiO 2  during firing; and   c) an organic medium.

FIELD OF THE INVENTION

This invention is related to thick film conductor compositions for useon LTCC (low temperature co-fired ceramic) tape that incorporate variousoxide additions to promote adhesion and the thermal stability of solderjoints under isothermal and temperature cycling conditions.

BACKGROUND OF THE INVENTION

An interconnect circuit board of an LTCC design is a physicalrealization of electronic circuits or subsystems made from a number ofextremely small circuit elements that are electrically and mechanicallyinterconnected. It is frequently desirable to combine these diverseelectronic components in an arrangement so that they can be physicallyisolated and mounted adjacent to one another in a single compact packageand electrically connected to each other and/or to common connectionsextending from the package.

Complex electronic circuits generally require that the circuit beconstructed of several layers of conductors separated by insulatingdielectric layers. The conductive layers are interconnected betweenlevels by electrically conductive pathways, called vias, through adielectric layer. Such a multilayer structure allows a circuit to bemore compact than traditional Al₂O₃ substrates by allowing verticalintegration.

LTCC tape has been widely used in the automotive and telecom industryfor its multilayer, cofiring and flexible design capabilities. One ofthe critical elements in the successful use of the materials is that thesurface conductors need to have good soldered adhesion under boththermal aging (isothermal storage at 150° C.) and thermal cycling(typically between a low temperature in the range of −55 to −40° C. anda high temperature in the range of 100–150° C.) conditions. Suchexposure often will result in the development of stresses in the solderjoints used to attach surface mounted components to the substrate. Theprimary reason for these stresses is a mismatch in thermal expansion ofthe various different materials that comprise the joint, namely, theceramic, the conductor metal, the solder metal, the metal that comprisesthe leads to the surface-mounted device and the material used to makethe device. Through careful layout design and skilled application ofunderfill materials one can distribute the stresses more evenly andprevent a concentration of stresses at any one joint. However, it is notpossible to eliminate these stresses completely. One key to sustaininggood soldered adhesion during such exposure is the ability of thematerials involved to absorb these stresses without undergoing anypermanent or irreversible mechanical damage. In other words anyimprovement in the mechanical properties, i.e., elastic and bulkmodulus, of the materials, particularly those of the underlying tapewill increase the ability of the overall joint to absorb stress duringthermal exposure.

One of the most common failure modes observed in the thermal cycling ofsolder joints is cracking of the tape at the periphery of the conductorpad. As exposure continues such cracks can propagate through thedielectric and under the pad itself. In some cases the onset of crackinghas been observed after less than 10 cycles. This is compared to atypical specification which requires 500 thermal cycles without evidenceof ceramic tape cracking or significant loss of adhesion between theconductor and the underlying tape.

U.S. Pat. No. 5,431,718 to Lombard et al. provides a high adhesionstrength, co-fireable, solderable silver metallization material for usewith low-fire ceramics. The metallization material includes the metalpowder as well as an organic vehicle, and adhesion promoting agents.This combination of elements allows a metallization material which canbe cofired at relatively low temperatures necessary for firing ceramicsubstrate materials while providing an adequate base for solderingsubsequent circuit components to the ceramic substrate.

U.S. Pat. No. 4,416,932 to Nair discloses a ceramic substrate having aconductive pattern coating wherein the coating comprises an admixture offinely divided particles of a noble metal or alloy, a low melting, lowviscosity glass, a spinel-forming metal oxide and an organo-titanate andthe process of making same.

Neither of the above inventions meets the specification of 500 cycleswithout cracking of the dielectric and maintaining adhesion of theconductor to the substrate during thermal exposure. Accordingly, thereexists a need for a conductor composition which is able to surpass thethermal cycling capabilities of the prior art. In particular, there is aneed for a conductor composition which can exceed 500 thermal cycleswithout cracking or losing adhesion on the tape substrate. The presentinvention provides such conductor compositions.

SUMMARY OF THE INVENTION

The invention is directed to a thick film composition comprising: (a) anelectrically conductive powder; (b) TiO₂ or any compounds that cangenerate TiO₂ during firing; and (c) an organic medium.

The invention is further directed to a method of forming a multilayercircuit comprising: (a) forming a patterned array of vias in a pluralityof layers of green tape; (b) filling the vias in the green tape layer(s)of step (a) with a thick film composition; (c) printing at least onepatterned thick film functional layer over a surface of each of thevia-filled green tape layers of step (b); (d) printing at least onepatterned layer of the thick film composition of the present inventionover the outermost surface of the green tape layers of step (c); (e)laminating the printed green tape layers of step (d) to form anassemblage comprising a plurality of unfired interconnected functionallayers separated by unfired green tape; and (f) cofiring the assemblageof step (e).

DETAILED DESCRIPTION OF INVENTION

The main components of the thick film conductor composition areconductor powders and titanium dioxide dispersed in an organic medium.The components are discussed herein below.

I. Inorganic Components

The inorganic components of the present invention comprise (1)electrically functional powders and (2) inorganic binder comprisingtitanium oxide, or any compounds that can generate TiO₂ during firing.The inorganic binder may further comprise additional inorganic oxidebinders.

A. Electrically Functional Powder

Generally, a thick film composition comprises a functional phase thatimparts appropriate electrically functional properties to thecomposition. The functional phase comprises electrically functionalpowders dispersed in an organic medium that acts as a carrier for thefunctional phase which forms the composition. The composition is firedto burn out the organic phase, activate the inorganic binder phase andto impart the electrically functional properties. Prior to firing, theprinted parts are dried to remove the volatile solvents. “Organics” is aterm used to describe the polymer or resin components of a thick filmcomposition, as well as solvents and small amounts of additional organiccomponents such as surfactants.

The electrically functional powders in the present thick filmcomposition are conductive powders and may comprise a single type ofmetal powder, mixtures of metal powders, alloys, or compounds of severalelements. The particle diameter and shape of the metal powder is notparticularly important as long as it is appropriate to the applicationmethod. Examples of such powders include gold, silver, platinum,palladium, and combinations thereof. The electrically functional powdersof the present invention have a typical size of D₅₀ less than about 10microns.

B. Inorganic Binder—Titanium Oxide

The titanium dioxide used in this invention has three possiblefunctions. First, it can act as a binder to provide adhesion of theconductor on the tape. Second, it could adjust the sintering rate of theconductor during firing to minimize the stress on the tape, and thethird, the presence of TiO₂ at the interface or trace amount of TiO₂diffusion into the tape could enhance the mechanical strength of thetape. The titanium oxide used in this invention could be directly in theform of an oxide or converted from Ti containing compounds under thefiring condition, such as elemental Ti, organo titanates, orcrystallization products from a frit. The Ti containing compounds can bebinary, ternary and higher. TiO₂ may be present in the total compositionin less than 2.0 wt. %. In one embodiment, the TiO₂ is present in therange of about 0.5 to 1.6 wt. % of the total composition. In a furtherembodiment, the TiO₂ is present in the amount of about 1.0 to about 1.6wt. % of the total composition.

Titanium dioxide was found to be very effective preventing the crackingof the tape during thermal cycling with addition levels in a very widerange. The general trend is that as the TiO₂ level increases, thermalaged adhesion will improve. However, solderability suffers when the TiO₂level is too high. The optimized level of the titanium oxide isdetermined by the extent that it does not cause poor solder wettingwhile at the same time providing sufficient adhesion under thermal cycleand thermal aging conditions. Titanium dioxide level also affects thedensity of the fired thick film and electrical conductivity.

C. Optional Inorganic Binder Components

Titanium dioxide, or any compounds that can generate TiO₂ during firing,were found to be the major and most effective components in thecomposition providing the functions of preserving the integrity of thetape and promoting thermal cycled adhesion. However, oxides such asCO₃O₄, PbO, Fe₂O₃, SnO₂, ZrO₂, Sb₂O₃, MnO_(x), CuO_(x) as well as otheroxides may also aid in adhesion to some degree, when combined withtitanium oxide.

D. Organic Medium

The inorganic components are typically mixed with an organic medium bymechanical mixing to form viscous compositions called “pastes”, havingsuitable consistency and rheology for printing. A wide variety of inertliquids can be used as organic medium. The organic medium must be one inwhich the inorganic components are dispersible with an adequate degreeof stability. The Theological properties of the medium must be such thatthey lend good application properties to the composition, including:stable dispersion of solids, appropriate viscosity and thixotropy forscreen printing, acceptable unfired “green” strength, appropriatewettability of the substrate and the paste solids, a good drying rate,and good firing properties. The organic medium is typically a solutionof polymer(s) in solvent(s). Additionally, a small amount of additives,such as surfactants, may be a part of the organic medium. The mostfrequently used polymer for this purpose is ethyl cellulose. Otherexamples of polymers include ethylhydroxyethyl cellulose, wood rosin,mixtures of ethyl cellulose and phenolic resins, polymethacrylates oflower alcohols, and monobutyl ether of ethylene glycol monoacetate canalso be used. The most widely used solvents found in thick filmcompositions are ester alcohols and terpenes such as alpha- orbeta-terpineol or mixtures thereof with other solvents such as kerosene,dibutylphthalate, butyl carbitol, butyl carbitol acetate, hexyleneglycol and high boiling alcohols and alcohol esters. In addition,volatile liquids for promoting rapid hardening after application on thesubstrate can be included in the vehicle. Various combinations of theseand other solvents are formulated to obtain the viscosity and volatilityrequirements desired.

The ratio of organic medium in the thick film composition to theinorganic components in the dispersion is dependent on the method ofapplying the paste and the kind of organic medium used, and it can vary.Usually, the dispersion will contain 50–95 wt % of inorganic componentsand 5–50 wt % of organic medium (vehicle) in order to obtain goodcoating.

APPLICATION

The conductor composition of the present invention may be used inconjunction with uncured ceramic material, such as Green Tape™, andvarious other paste components, to form a multilayer electronic circuit.Green Tape™ is typically used as a dielectric or insulating material formultilayer electronic circuits. The uncured ceramic material may by alead free tape; for example one embodiment is disclosed in PatentCooperation Treaty International Application Number PCT/US 03/17255,herein incorporated by reference. One embodiment of a lead free tapeuseful in the present invention is identified as Product number 960 byE. I. du Pont de Nemours and Company. In one embodiment, the lead freegreen tape comprises a glass composition comprising, in mole %, 2–8% M,wherein M is selected from an oxide of the group of alkali elements andmixtures thereof, 46–66% SiO₂, 3–9% Al₂O₃, 5–9% B₂O₃, 0–8% MgO, 1–6% SrOand 11–22% CaO wherein the molar ratio of SrO/(MgO+CaO) is between 0.06and 0.45. In a further embodiment, the glass composition above isutilized except that SiO₂ is partially replaced by oxides selected fromthe group of ZrO₂, P₂O₅, GeO₂ and mixtures thereof within the limits,0–4% ZrO₂, 0–2% P₂O₅, 0–1.5% GeO₂ in mole % of the total composition. Instill a further embodiment, the initial glass composition above isutilized 1 wherein SrO, MgO and CaO are partially replaced by CuO withthe proviso that CuO is 0–2.5 mole % of the total glass composition.

A sheet of Green Tape™ is blanked with registration holes in each cornerto a size somewhat larger than the actual dimensions of the circuit. Toconnect various layers of the multilayer circuit, via holes are formedin the Green Tape™. This is typically done by mechanical punching,however, any suitable method may be utilized. For example, a sharplyfocused laser can be used to volatilize and form via holes in the GreenTape™.

The interconnections between layers are formed by filling the vias witha thick film conductive composition. In the case of this invention, athick film conductive composition different to that disclosed herein istypically utilized. This conductive composition is usually applied bystandard screen printing techniques, however, any suitable applicationtechnique may be employed. Each layer of circuitry is typicallycompleted by screen printing conductor tracks. Also, resistor inks orhigh dielectric constant inks can be printed on selected layer(s) toform resistive or capacitive circuit elements. Conductors, resistors,capacitors and any other components are typically formed by conventionalscreen printing techniques.

The conductor composition of the present invention may be printed on theoutermost layers of the circuit, either before or after lamination. Theoutermost layers of the circuit are used to attach components.Components are typically wire-bonded, glued or soldered to the surfaceof fired parts. In the case of a soldered component, the conductorcomposition of the present invention is particularly useful as it mayhave superior thermal aged and thermal cycle adhesion over prior artcompositions.

After each layer of the circuit is completed, the individual layers arecollated and laminated. A confined uniaxial or isostatic pressing die istypically used to ensure precise alignment between layers. Theassemblies are trimmed to an appropriate size after lamination. Firingis typically carried out in a conveyor belt furnace or in a box furnacewith a programmed heating cycle. The tape may be either constrained orfree sintered during the firing process. For example, the methodsdisclosed in U.S. Pat. No. 4,654,095 to Steinberg and U.S. Pat. No.5,254,191 to Mikeska may be utilized, as well as others known to thoseskilled in the art.

As used herein, the term “firing” means heating the assembly in anoxidizing atmosphere, such as air to a temperature, and for a timesufficient to volatilize (burn-out) the organic material in the layersof the assemblage and allow reaction and sintering of the inorganiccomponents of both the tape and conductors. “Firing” causes theinorganic components in the layers, to react or sinter, and thus densifythe entire assembly, thus forming a fired article. This fired articlemay be a multilayered circuit used in telecom and automotiveapplications (such as automotive vehicles).

The term “functional layer” refers to the printed Green Tapee™, whichhas conductive, resistive, capacitive or dielectric functionality. Thus,as indicated above, a typical Green Tape™ layer may have contain one ormore conductive traces, conductive vias, resistors and/or capacitors.

EXAMPLES

The invention will now be described in further detail with practicalexamples (Examples 1–9) and a comparative example (Example 10). In theseexamples, the paste component and their percentages are listed in Table1.

The paste of Examples 1–10 was formed according to the followingmethodology as detailed by the preparation of Example 1.

The organic mixture was made by dissolving 15 g ethylcellulose in 85 gof an appropriate solvent, such as terpineol.

A mixture of 17–18 g of the above mentioned organic vehicle and 4–5 gsolvent was combined with 1.9–2.1 g platinum powder, 1.4 g titaniumdioxide, and 74–75 g silver powder. This mixture was thoroughlydispersed using a three-roll mill. The paste was then formulated withthe appropriate amounts of the medium or solvent disclosed above toachieve a viscosity of 150–400 PaS at 10 rpm as measured on a BrookfieldHBT viscometer.

Test Procedures Used in Examples

Conductor paste was printed on Du Pont 951 or lead free (ref patent)tape, dried at 120° C. for 5–10 minutes and laminated to the requiredthickness and layer count to make a 0.050 inch fired thick part. Thetape was then cut into the appropriate size to make 1×1 inch parts andfired under standard firing condition for Du Pont Green Tape™ (850° C.for 3.5 h).

Soldered Adhesive Strength Test Method

A typical test pattern is printed with a conductor resistivity testpattern and nine 0.080×0.080 inch pads. Fired thickness of the conductorwas between 14 to 17 μm. None of the conductor compositions describedherein showed observable distortion on the tape.

For all adhesion tests, three clip-like wires were attached across eachrow of 3 pads and dip soldered. The 951 Green Tape™ parts were solderedat a temperature of 220° C.±5° C. for five seconds using 62/36/2(Sn/Pb/Ag) solder. The parts made on lead free tape were soldered at260° C.±5° C. using 95.5/3.8/0.7 (Sn/Ag/Cu) solder. After soldering,residual solder flux was cleaned from the soldered wire parts withArcosolve. The parts were then divided into individual test samples forinitial adhesion, thermal aged adhesion (150° C. soak) or thermal cycledadhesion (−40 to 125° C., 2 hour per cycle). A sample set consists ofthree to four parts for each test condition.

The parts are allowed to rest at room temperature for 16 hours afterbeing soldered or after being removed from the relevant thermal chamber.For adhesion testing, the wire leads were bent to 90° in accord with thebending marks printed on each part and tensile strength was measured foreach pad. The average of three (3) pads per part for three to four partsas measured was used as the adhesive strength of the thick filmconductor applied to the substrate. This format was used for alladhesion testing—initial, thermal aged and thermal cycled.

Thermal Aged Adhesion

For this test, parts were placed in an oven at 150° C., and removed atapproximately the following intervals (hours) for testing: 48, 100, 250,500, 750, 1000.

Aged adhesion was measured according to the test method described above.For each test, the type of separation was noted, i.e., whetherseparation involved the wires pulling out of the solder, conductorpulling off the substrate, or cracking in the substrate. Any adhesionvalue above 12N was considered acceptable—any value below 10N wasconsidered unacceptable.

Thermal Cycling and Thermal Cycling Adhesion

After soldering and cleaning, parts were placed in a thermal-cyclechamber which was then cycled between −40 and 125° C. every two hours.Parts were taken out for testing at different intervals (cycles). Thenominal number of cycles chosen was 10, 30, 100, 250, and 500. Theactual intervals chosen varied and the choice depended on the nature ofthe intermediate test results obtained.

The parts were examined under the microscope to check for any signs ofcracking on tape or in solder before the wires were pulled for adhesion.After adhesion testing as described above, the failure mode combinedwith adhesion value was used to evaluate acceptability. If a tapefracture was combined with a very low adhesion value, it indicated thatthe tape underneath was cracked, even if cracks were not visuallyobserved. If cracking was visually observed in the ceramic, the part waslabeled as having failed this test. As before, failure without visualcracks was defined as adhesion below 12N.

Solder Wetting

The test pattern with no leads attached was immersed for 5 sec at 220°C. in solder. After solder application, the state of solder coverage wasobserved.

Measurement was carried out by the ratio of the pads to which solder hadadhered completely, so that the conductor surface not visible to thetotal number of pads. In the case of this invention, 90% or above isconsidered good.

Example 10 is a control using a state of the current art Ag/Pdcomposition. This composition fails very early due to cracks in theceramic during thermal cycling. This composition also exhibits marginalthermal cycled and aged adhesion.

Examples 1–4 show the effect of different titania concentrations overthe claimed range. As the titania level is increased, thermal cycledadhesion improves. Thermal aged adhesion and solder wetting show amaximum value around 1.4% titania.

Example 5 shows the effect of adding both titania and antimony oxide tothe composition. This addition shows an improvement in thermal cycledand thermal aged adhesion versus the control.

Examples 6–9 show the effect of adding both titania and cobalt oxide tothe composition. This data shows that thermal cycled adhesion isdramatically improved over the control composition.

Example 10 shows the effect of using both titania and antimony oxide onlead free tape dielectric.

Conductors 1–10 were made using 74–75% Ag, 1.9–2.1% Pt dispersed in20–22% organic vehicle and solvent.

Thermal- Isothermal Soldera- cycled aged bility TiO2 Co3O4 Sb2O3adhesion adhesion % cov- Percent by weight # cycles Adhesion (N) erage 11.4 0 0 >500 >12 >12 >90 2 1.6 0 0 >500 >12 >12 <90 3 1.1 00 >500 >12 >12 <90 4 0.6 0 0 >500 >12 <12 >90 5 0.8 00.5 >500 >12 >12 >90 6 0.6 0.1 0 >500 >12 <12 — 7 0.6 0.3 0 >500 >12 <12— 8 0.8 0.1 0 >500 >12 <12 — 9 0.1 0.6 0 400 >12 <12 — 10 0.7 00.7 >500 >12 >12 >90 Control: commercial 50 <12 >12 >90 solderable Ag—Pdconductor

1. A thick film composition consisting of: a) electrically conductive powder; b) an inorganic binder wherein the inorganic binder is TiO₂ and a minor amount of one or more oxides selected from the group of oxides consisting of CO₃O₄, PbO, Fe₂O₃, SnO₂, ZrO₂, Sb₂O₃, MnO_(x), CuO_(x) and mixtures thereof, combined with said TiO₂ and c) an organic medium.
 2. The composition of claim 1 wherein said electrically conductive powder is selected from gold, silver, platinum, palladium, and mixtures and alloys thereof.
 3. The composition of claim 1 wherein the total inorganic binder is in the range of 0.5 wt. % to about 1.6 wt. % of the total composition.
 4. The composition of claim 1 wherein said composition is compatible with low temperature co-fired ceramic tape.
 5. A multilayer circuit formed by the method of claim
 1. 6. A multilayer circuit utilizing the composition of claim
 2. 7. The composition of claim 1 wherein said composition is applied to a dielectric green tape layer and wherein said thick film composition and dielectric green tape layer are thermally processed to produce a conductor layer and a fired dielectric tape layer wherein said conductor layer is capable of exceeding 500 cycles without cracking said dielectric tape layer and maintaining adhesion of said conductor to said fired dielectric tape layer. 